Back pressure regulator



FIG. 1 is a right side elevational view of a back pressure regulator showing my new design, the left side being a mirror image of the right side shown;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a top plan view thereof; and

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a back pressure regulator, substantially as shown and described. 